1. Field of the Invention
This invention relates to a method and apparatus for isolating a portion of a wellhead during a fracturing operation.
2. Description of the Related Art
A typical oilfield well comprises several strings or tubing, such as casing strings. FIG. 1 illustrates one particular conventional well. The illustrated well includes a casing head 10 supporting an outer casing string 15. A casing hanger 20 is landed in the casing head 10 and supports an inner or production casing string 25. A tubing head 30 is disposed above the casing head 10. During normal production operations, the tubing head 30 supports a tubing hanger (not shown) and production tubing (also not shown). The production casing string 25 extends downward into a hydrocarbon bearing formation 35.
It is common in oilfield production operations to “workover” a slow producing or marginal well to stimulate and increase production. Such workover techniques may include high-pressure fracturing of the formation 35, known to the art as “fracing” a well or formation. It is also common to fracture a new well to increase the production capability of the well. Generally, in this process, a sand-bearing slurry is pumped down into the formation at very high pressures. The sand particles become embedded in small cracks and fissures in the formation, wedging them open and, thus, increasing the flow of produced fluid. Such fracturing processes are typically more efficient at lower portions of the wellbore 40.
For example, as illustrated in FIG. 1, fluid may be pumped into the production casing 25, achieving an efficient fracture of the lowest zone 45. A bridge plug 50 may then be installed above the lowest zone 45, after which the well is fractured again, achieving an efficient fracture of the middle zone 55. A second bridge plug 60 may then be installed above the middle zone 55, after which the well is once again fractured, achieving an efficient fracture of the upper zone 65. The bridge plugs 50, 60 are typically installed using a wireline lubricator. While three zones (e.g., the zones 45, 55, 65) are illustrated in FIG. 1, any number of zones may be identified in a well and any number of fracturing cycles may be performed.
The tubing head 30 and any valves associated with the tubing head, such as a valve 70 in FIG. 1, are typically rated for the expected formation pressure, i.e., the pressure of fluids produced from the well. The fracturing pressure, however, is typically much higher than the formation pressure and often exceeds the pressure rating of the tubing head and valves. Moreover, the fluids used during fracturing are often very abrasive and/or corrosive. Therefore, the tubing head 30 and other such components of the top flange connection 78 are often isolated and protected from the fracturing fluid by a wellhead isolation tool 75. A conventional wellhead isolation tool 75 mounts above a frac tree assembly 80 and comprises an elongated, tubular stab that passes through the tubing head 30 and seals to the inside surface of the production casing 25. The fracturing fluid may then be pumped through the wellhead isolation tool 75, bypassing the tubing head 30 and frac tree assembly 80. Thus, the flange connections between the tubing head 30, the frac tree assembly 80 and tubing head annulus gate valves 70 are isolated from the pressure and the abrasive/corrosive characteristics of the fracturing fluid.
One difficulty that arises in this arrangement is that the inside diameter of the wellhead isolation tool 75 is substantially smaller than the inside diameter of the casing string 25, because the wellhead isolation tool 75 seals to the inside surface of the casing string 25. FIG. 1 illustrates the inside radius A of the wellhead isolation tool 75 is smaller than the inside radius B of the casing string 25. Since the outside diameter of the bridge plugs 50, 60 (or any downhole plug/tool), are substantially the same as the drift of the casing string 25, the bridge plugs 50, 60 cannot pass through the wellhead isolation tool 75. Therefore, each time a bridge plug 50, 60 is installed, the wellhead isolation tool 75 must be removed and the wireline lubricator installed. After installing each bridge plug 50, 60, the wireline lubricator is removed and the wellhead isolation tool 75 is reinstalled for the next fracturing cycle. This repetitive installation and removal of equipment adds significant cost and time to the management of the well.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.